Program Summary Infectious diseases including chronic infections such as hepatitis B virus (HBV) remain leading causes of death worldwide. Influenza (Flu) and other respiratory infections kill ~half a million people globally each year, and our vaccines for many infections remain poorly effective, especially in vulnerable populations. Most fundamental insights into the mechanisms of immune responses to pathogens have come from animal models. Interrogating human immune system function has typically been limited to descriptive analyses or confirmation of observations from animal studies. This gap in knowledge limits our ability to use rational manipulation of specific immune pathways to effectively combat chronic infections and improve vaccines. The ongoing revolution in cancer immunotherapy ? in which blockade of immune checkpoints such as PD-1 is used to stimulate cancer immunity ? has not yet been exploited for human chronic infections and vaccines. Here we will test how PD-1 pathway blockade alters immunity in humans to chronic infections or vaccination, providing an opportunity to dissect the mechanistic role of this pathway in human innate and adaptive immunity. The studies proposed are highly innovative and have potential to reveal groundbreaking new fundamental knowledge of human immunity, because a) the PD-1 pathway is central to T and B cell responses to pathogens; and b) there is almost no information in humans about how blocking PD-1 affects cellular or humoral immunity to chronic viral infection or vaccines. Thus, the central goal of this Program is to employ specific immune perturbation through PD-1 blockade during persistent HBV infection and Flu vaccination, and use this intervention to define the innate and adaptive immune mechanisms controlled by PD-1 for improved therapeutic and preventative treatments for human infections. This Program will develop a broad platform with which to interrogate other existing or emerging immunotherapies and human immunity to infection. Thus, our U19 Program is organized around the following central hypothesis: ?We hypothesize that perturbing specific immunoregulatory pathways (e.g., PD-1) during human persisting viral infection and vaccination will provide insights into fundamental immune mechanisms involved in pathogen immunity and lead to novel opportunities for therapeutics and vaccines.? We will capitalize on this revolution in human immune system perturbation ? moved to the clinic initially for cancer ? to understand basic mechanisms of immune responses to persisting infections and vaccines. We propose 2 Projects to: 1) define how PD-1 blockade in humans re-invigorates HBV-specific CD8 T cell and bystander immune responses in HBV+ patients; and 2) test how PD-1 blockade impacts vaccine-induced CD4 T cell, B cell and antibody responses. The Projects are supported by administrative and scientific Cores to enable clinical sample acquisition, antibody evaluation and TCR, BCR and single-cell RNA sequencing.